Adding ability to restrict allowed characters in an asset id and allowing ( and )

[JackLewis-digirati]

Resolves #340
Resolves #323

This pull request adds in the ability to restrict the characters allowed in an asset id. It does this by adding a set of 2 new settings:

"RestrictedAssetIdCharacters": "\\ "
"DisableStrictAssetIdChecks": true

RestrictedAssetIdCharacters is a series of characters that are not allowed within an asset id

DisableStrictAssetIdChecks sets whether the check for strict asset ids is disabled. As such, this check is enabled by default

Additionally, this pull request enables ( and ) characters to be used in the image id which would previously cause appetiser to fail. It manages this by replacing these characters with an alternative. By default this is an underscore, but can be set to a specific string via settings to be able to reduce the possibility of collisions

[donaldgray]

This doesn't appear to meet the criteria on #340.

Configuration includes a string that contains the permitted characters

This has a configuration that stores restricted characters

and also a flag to enable:
no restrictions
restriction for new assets but allow updates of existing
fail for updates too

This has a single boolean flag to enable/disable and doesn't appear to differentiate creation/update

[donaldgray]

Approved - just need to delete unused param